1. Field of the Invention
The present invention relates to an apparatus for supplying AC power to a commercial power line by using sunlight. More particularly, the invention relates to an apparatus for supplying AC power to a commercial power line by using sunlight wherein the output power of a solar battery is boosted and then converted into AC power.
2. Description of the Related Art
Recently, attention has been given to a system for interconnecting other systems which connects an apparatus for supplying AC power to a commercial power line by using sunlight. In these other systems, DC power is outputted from a solar battery which is converted into AC power similar to commercial power by an inverter circuit or the like. Thus, a commercial power system may include solar power for household electric appliances and the like.
Some conventional apparatuses for supplying AC power to a commercial power line in an interconnection system boost the voltage of DC power outputted from a solar battery by a booster circuit such as a boosting chopper circuit. Then the boosted DC power into AC power with an inverter circuit. At this point, in the conventional apparatuses for generating electric power by using sunlight described above, the booster circuit is controlled in such a way that the voltage after boosting is a fixed value.
However, a problem exists in that the operating voltage of the solar battery is unstable when the generated power of the solar battery is relatively low.
The problem is due to a high boosting rate set by the booster circuit when the output voltage of the solar battery is low.
More particularly the output voltage-output current characteristic of the solar battery is represented by a curve as shown in FIG. 6. Therefore, as the boosting rate of the booster circuit increases, the operating voltage of the solar battery shifts lower (to the left in FIG. 6), resulting in an unstable operating voltage for the solar battery.
Further, if the output voltage of the booster circuit, which is controlled in such a way that it is a fixed value, is set higher than necessary, there also arises a problem in that the conversion efficiency of the apparatus is decreased.